Cohort Analysis and Population Dynamics of Large Yellow Croaker in the China Sea

Cohort analysis is used to reconstruct the population dynamics of the large yellow croaker (Pseudosciaena crocea (Richardson)) in the China Sea, using age-composition and catch data from 1957 to 1979. A precondition for cohort analysis is that natural mortality M and terminal catchability coefficient q be known. These are estimated from age-composition data for two relatively stable periods with widely different exploitation rates. Cohort analysis suggested the virginal stock size of large yellow croaker was about 730,000 t. The biomass remained at a high stable level (about 480,000 t) until the middle 1960's; the stock then decreased rapidly because of increasing fishing effort. Recruitment apparently increased at first as the stock declined, but there are signs of decreased recruitment at the current low stock size. The present stock is about 200,000 t, the lowest on record. Different recovery and management strategies were evaluated by computer simulation. The maximum equilibrium yield is probably 95,000-100,000 t, associated with an equilibrium biomass of 390,000 t. Considering environmental effects on recruitment, the yield is likely to fluctuate between 70,000 and 130,000 t. To maintain the maximum average yield, the fishing effort should be equivalent to approximately the present annual fishing capacity of 1,000 pairs of 60-horsepower, paired trawlers in the Zhowshon district. About 15 years will be needed for the stock to recover to optimum equilibrium biomass if the optimum long-term effort is maintained.